For many years alternating current (a.c.) power was measured by wattmeters of the dynamometer type. Such meters employed electromagnetic coils which moved a needle or disk to indicate the average power. It was found, however, that such instruments were not entirely accurate, were slow in response and did not provide a direct current output useful in recording and testing instruments.
In recent years there have been many proposals for watt transducers and wattmeters of the electronic type. Some of these electronic watt transducers utilize Hall Effect devices, which may be relatively costly and inaccurate and require elaborate temperature compensation. Other such proposals for electronic watt transducers are based upon statistical coincidence methods and employ complex circuitry, such as those of U.S. Pats. Nos. 3,510,772 and 3,525,042. Other electronic wattmeter instruments, such as those of U.S. Pats. Nos. 2,879,477 and 3,662,264, utilize circuitry to, in effect, compute the square law to determine the average power.
The present invention, however, utilizes a type of electronic watt transducer in which one waveform provides the control to a switch which is connected to transmit the other waveform. This general type of wattmeter is described in U.S. Pats. Nos. 3,794,917 and 3,500,200. In U.S. Pat. No. 3,500,200 a triangular waveform is modulated by the current input to have a variable width, then modulated by the voltage input as to amplitude, and the pulses integrated to produce a d.c. output representative of the a.c. average power consumed. In U.S. Pat. No. 3,794,917 the voltage input and a precision triangular waveform are coupled to a comparator which controls electronic switches, so that the duty ratio of the output of the comparator is proportional to the voltage amplitude. However, the circuitry of these two patents is relatively complex and costly. The complexity of their circuitry makes it difficult to guard against the adverse effects of aging and temperature variations, and consequently may lead to inaccuracy of measurement.
Wattmeters measure the average electric power when the current and voltage have common frequency, although they may have a phase difference between them. The average power W to be representative of true power consumed must take into account the voltage, the current and the difference between their wave peaks (phase displacement .theta.) which phase displacement is the "power factor" represented by cos .theta.. The average power W at a given frequency is the product of the rms values of voltage V, current I and the cosine of phase difference .theta., i.e., W = VI cos .theta.. In general, the average power W, over an interval T, is equal to the integral ##EQU1## where e and i are the instantaneous values of voltage and current. The wattmeter of the present invention produces an output signal which closely approximates that integral equal to W. A pulse train is modulated so that the duty cycle of the pulses are proportional to I and the amplitude of the pulses are proportional to V. When this signal is filtered, the d.c. voltage resulting closely approximates W.